The present invention relates to a heating resistor type air flow measurement apparatus for measuring air flow rate by using an amount of heat carried off by air from a resistor arranged in the airflow, and particularly to an air flow measurement apparatus suitable for the measurement of intake air flow in an internal combustion engine for automobiles.
Since a heating resistor type air flow measurement apparatus is superior in that it has no movable parts, and it is possible directly to detect a mass air flow rate, and further it is easy to miniaturize and save its weight. As a result, it is coming in for use in engine control for automobiles.
Recently, various kinds of plastic members have been developed. With the improvement, some components of the engine or the automobile are converted from the conventional metal-made components to the plastic-made. In recent years, the manufacturing cost and the product weight has been extremely reduced.
Under such a technical flow, the body of the air flow meter is also made of plastic member, instead of the metal-made.
As a result, newly one problem on electromagnetic interference has occurred. Namely, because in general a flow measurement circuit handles with a relatively feeble voltage, it is very susceptible to electromagnetic noises. There is, therefore, a strong fear that the malfunction may occur.
While, in an engine room of an automobile, the intensity of electromagnetic noises is relatively large. Therefore, the flow measurement circuit is susceptible to the electromagnetic noises.
In the case that the body of a main passage is metal-made, the electromagnetic shielding effect is large due to the conductivity of the body. The electromagnetic interference is thus reduced. While, in the case that the body is plastic-made, the problem of the electromagnetic interference is occurred, since the electromagnetic shielding effect is hardly expected.
For example, in the Japanese Patent Application Laid-Open No. 8-5425 (1996), such technique is disclosed, that characteristic of electromagnetic interference is provided by using a circuit having a low-pass filter between a heating resistor and a temperature-sensitive resistor, and a flow detection circuit.
In the prior art in which such a low-pass filter is provided, the effect of the capacitor of the filter is not taken into consideration. However, a noise current passing through the capacitor occurs when a large surge current caused by ignition noises flows. Thereby, the flow measurement circuit may fail to work properly.
Namely, the malfunction due to a normal radio frequency field is caused by noises induced mainly in the heating resistor and the temperature-sensitive resistor. With regard to such malfunction, it is possible to provide a necessary characteristic of the electromagnetic-interference-resistance to the air flow measurement apparatus by providing a low-pass filter as described above. On the contrary, when a large surge current is produced by the ignition noises, a large noise current passes through a capacitor of the low-pass filter, and the malfunction is thus induced.